1. Field of the Invention
The present invention is related to an apparatus and a method for determining the performance of micromachined or microelectromechanical devices (MEMS) with movable parts that handle high frequency signals. More specifically the performance parameters related to the moving behavior of such movable devices are investigated. These movable devices are electromechanical movable devices such as MEMS switches, relays, varactors, tunable capacitors or similar.
2. Description of the Related Technology
Micromachined or microelectromechanical devices can be found in a large number of applications including sensors, actuators and transducers. An example of such a field of application is wireless communication, which is expanding at an incredible pace for applications ranging from mobile phones to satellite communications. “RF MEMS” technologies are central to many parts of this expansion. The term “RF MEMS devices” designated a variety of micromachined or MEMS devices such as tunable capacitors or microswitches used in these telecommunication circuits and as such these MEMS devices are operative in a given high frequency range. These RF-MEMS components are expected to be a solution to most of the off-chip components required by state of the art high frequency transmitter and receiver systems, like antenna switches, off-chip capacitors and filters, whether tunable or not. The electrical behavior, e.g. the capacitance value, the making of an electrical contact, of some of these “RF MEMS” devices can be tuned as these devices comprise a movable part. The ability to tune or to change the electrical characteristics of such micromachined device allows the development of reconfigurable circuits: one can alter the operating range or the selectivity of the electrical circuit by e.g. changing the capacitance value of the capacitors used in a filter, by e.g. selecting components by means of a switch. The moving action of the device can be controlled by means of an applied voltage resulting in an electrostatic force.
However before this “RF-MEMS” technology can be implemented into mainstream systems the lifetime, reliability and endurance of these movable electromechanical devices must be demonstrated. Parameters related to the switching or moving action of these MEMS devices are e.g. pull-in voltage, rise- and fall-time of the switching action, on-capacitance, off-capacitance, and drift in any of these parameters. Nowadays these devices are tested at their intended operation or signal frequency (the frequency of normal operation for which the device was designed) which can be as high as 100 GHz, thereby requiring the use of complex and expensive testing tools. These testing tools need calibration and a controlled test environment, which makes it difficult to determine the characteristics of the MEMS device over the full range of the military specifications (MILSPEC). Given such testing equipment only a limited number of RF MEMS devices, most often only one, can be tested simultaneously, making it impractical to collect the large number of data needed to establish a good statistics of the characteristics of the RF MEMS device under study. In “lifetime characterization of capacitive RF MEMS switches”, published on pp 227–300 of the Proceedings of the May 2001 IEEE International Microwave Symposium held in Arizona, C. Goldsmith et Al. report on the testing of a capacitive coupled RF MEMS switch with a lifetime of 5 108 cycles. The authors use a dual-pulse actuation voltage to reduce the high voltage portion of the actuation signal and a 10 GHz continuous wave, representative for a realistic signal wave as present during operation of the device. This 10 GHz signal is being multiplexed with this dual-pulse actuation voltage. The results were obtained by testing one-by-one the devices at the intended working frequency. The measurements were hence expensive and time-consuming and statistically relevant lifetime distribution cannot be obtained given the limited number of devices tested.
Accordingly, there is a need for the development of cheap lifetime testing equipment, which allows a time-efficient and statistical relevant measurement of the lifetime distribution of high frequency micromachined devices, such a solution would have a large commercial value.
In U.S. Pat. No. 5,506,454 an apparatus for self-diagnosing the characteristics of an acceleration sensor used in for instance the automotive industry, and a method for the diagnosis thereof are disclosed. The self-test comprises the application of a force corresponding to a predetermined acceleration of a mass part, by applying a corresponding signal for diagnosis to the acceleration sensor. The application of this force and thus corresponding signal for diagnosis can simulate the normal working of the device during acceleration, while the device is further left under normal operation conditions. No problem is mentioned related to the testing of high frequency micromachined devices. Commercial accelerometers carry signals of typically 1 to 50 kHz.